Wire twister

ABSTRACT

A wire twister having a pre-twister and an infinitely variable positive drive that provides very precise control of the ratio of the velocity of the pre-twister to the velocity of a primary twister to achieve a substantially &#39;&#39;&#39;&#39;dead&#39;&#39;&#39;&#39; twist of ferrous metallic strands. The drive for the pre-twister uses tachometer generators to provide signals indicative of the pre-twister and primary twister velocities.

United States Patent 151 3,685,27 1 Wall et al. [451 Aug. 22, 1972 [54]WIRE TWISTER 2,526,247 10/1950 Lewis ..57/70 72 Inventors: Charles A w NL tz 2,788,632 4/ 1957 Dewhirst ..57/68 1 both ofNonhfila, g' 3,413,79312/1968 Stine et a] .......57/68 [73] Assignee: The Warner SwaseyCompany, I primary E i j h petrakes Cleveland Attorney-Yount and Tarolli[22] Filed: May 14, 1971 57 ABS CT [21] Appl. No.: 143,416 1 A wiretwister having a pre-twister and an infinitely variable positive drivethat provides very-precise confi S 57/34 57/70 v trol of the ratio ofthe velocity of the pre-twister to the l 'P i velocity of a primarytwister to achieve a substantially 1 o arc q fdead twist of ferrousmetallic strands. The drive for the pre-twister uses tachometergenerators to provide [56] References Cited signals indicative of thepre-twister and primary UNITED STATES PATENTS t t velocities- 2,484,l7910/1949 MacCreadie ...57/68 6 Claim, 6 Drawing Figures PATENTEDmczz m23.685.271 sum 1 or 2 /VENTUR5 I EHA'RLEE. A. WAL

F. N DR'MAN LLJTZ By M ATTOE/WEY;

3,685,271 sum 20? 2 PATENTEDmczz I972 WIRE TWISTER BACKGROUND OF THEINVENTION This invention relates generally to twisters or stranders, andmore particularly to twisters having a pretwister.

.Twisters and stranders have been known and used for many years in themanufacture of rope, stranded wire, and cable. Many stranding operationsmay be performed on either a strander or a twister, but some operations,for example stranding ferrous wire, have been extremely difficult toperform on a twister. This is because a twister imparts twist to theindividual wires whereas a strander does not. However, stranders arerestricted in the number of wires they can strand, and may requirespecial reels for holding the wire to be stranded. Twisters, on theother hand, are quite versatile and may be used to twist a large numberof different combinations of wire size and number of wires. Therefore,it is greatly desirable to extend the ability of the twister to includetwisting ferrous wire strands and thereby eliminate the need for lessversatile and more expensive stranders.

Because ferrous metals have a high modulus of elasticity and a highyield point, each wire in a strand twisted on a twister will betorsionally displaced and will tend to spring back to its originalshape. This causes the strand to be live and tend to kink and twist uponitself. In order to provide 'a dead strand of ferrous wires with atwister, it is necessary to stress the individual wires beyond the yieldpoint to cause permanent deformation. That is, the strand must beovertwisted and then relaxed so that there are no residual torsionalforces in the wires when the twisting torque is completely removed.

It is not enough, however, to merely achieve dead strand. In order to beuseful as, for example, tire cord, the strand must be very uniform. Thismeans that, among other things, the lay of the strand, i.e., the lengthof each full twist measured along the strand, must be very uniform.Therefore, deadness cannot be achieved by simply letting the twistedstrand relax until it is dead, because this would change the lay.Instead, it is mandatory that deadness be achieved with a uniform andcontrollable lay.

In US. Pat. No. 2,526,247 issued Oct. 17, 1950 to Dartrey Lewis forMethod and Apparatus for Producing Wire Strandor Rope, there isdisclosed the method of overtwisting and then untwisting the strand toproduce any desired residual twist in either direction. This method iscarried out by a tandem twisting arrangement comprising a first twistinghead (pretwister) and a secondtwisting head and flyer (primary twister).The pre-twister is operated at a higher velocity than the primarytwister, thus providing overtwist and subsequent untwisting. The Lewisapparatus apparently works well for rope and nonferrous metal wire, buthas been unable to twist high strength ferrous wire with the degree ofdeadness and uniformity required for tire cord.

We have determined that the inability of the Lewis apparatus to providesatisfactory high strength ferrous strands did not lie with the basicmethod of overtwisting and relaxing or untwisting, but lay primarilywith the variable speed drive for the pre-twister. It was determinedthat mechanical variable speed drives of ble, would not maintain theirsettings, but varied over time and with load due to slippage. Mechanicaldrives of the type having gears, teeth, or cogs which positively engageprovide good time and load stability, but are incapable of fine enoughadjustment to provide the required degree of' deadness. Therefore, wehave developed a velocity ratio drive for the pre-twister that has aresolution of one revolution per minute throughout the range of 0-4,000RPM.

Also because the proper velocity ratio between the pre-twister and theprimary twister will vary slightly from reel to reel of wire to betwisted, the required deadness cannot be achieved where more than onepretwister is driven by the same variable/speed drive as suggested byLewis. This, of course, does not prevent a plurality of primary twistersfrom being mounted on a common frame and being driven by a common drive.It does, however, require'that the ratio of the velocity of eachpre-twister to the velocity of its primary twister be individuallycontrollable.

OBJECTS AND SUMMARY OF THE INVENTION An object of the present inventionis to provide an improved apparatus for twisting ferrous metallic wireinto a strand having substantially'no residual twist.

It is a further object of the presentinvention to provide an improvedvelocity ratio control for driving a pre-twister at a desired velocityratio with respect to a primary twister to produce a twisted strandhaving the desired deadness.

' It is still another object of the present invention to provide atwisting apparatus wherein a plurality of primary twisters are mountedon a common frame and are commonly driven by a primary drive and whereinan equal number of pre-twisters are mounted on the same frame but eachpre-twister is provided with its own adjustable variable speed drive.

The foregoing and other objects of the invention are achieved bygenerating electrical signals proportional to the velocities of eachprimary twister and each pretwister, and using these signals to generatecontrol signals for controlling the velocity of the pre-twisters so thateach remains in a preselected ratio to the velocity of its associatedprimary twister.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a generalized perspectiveview of a fourspindle twister embodying the present invention.

FIG. 2 is a diagrammatic cross-sectional view of one spindle withassociated creel, pre-twister, flyer, and take-up reel.

FIG. 3 is a block diagram of the servo control for driving apre-twister.

FIG. 4 is a graphical illustration of the transfer function of apre-twister and the motor used to drive it.

FIG. 5 is a schematic diagram of the preferred laglead compensationnetwork used in the servo control of FIG. 3.

FIG. 6 is a graphical illustration of the transfer function of thecompensation network of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT 14, and top members 15.Supported by the frame 12 is a housing 21 which houses a conventionalspindle drive that rotates and axially reciprocates take-up shafts 22.Each take-up shaft 22 can support a take-up reel 23. Because theillustrated twister of FIG. 1 has four spindles, there are, of course,four take-up shafts 22. Only two of the take-up shafts 22 are visible inFIG. 1, the other two being obscured by a partition 24. I

Mounted on end members 14 are four main spindle and flyer assemblies 25,each of which is coaxial with its associated take-up shaft 22. Alsosupported by end members 14 are four pre-twister assemblies 27 mountedto the end members 14 by brackets 28. Each of the pre-twister assemblies27 is coaxial with its associated main spindle and flyer assembly 25 andtakeup shaft 22. Each pre-twister assembly is provided with c aprotective housing 29.

FIG. 2 shows in greater detail the component parts and relationshipsbetween an associated pre-twister assembly 27, main spindle and flyerassembly. 25, and take-up shaft 22. Hereafter, the main spindle andflyer assembly 25 may also be referred to as the primary twister. Wireto be twisted is supplied to the twister from reels 34 on a creel 35.Creel 35 is conventional, having a tensioning device and stop motion,indicated at 36, for each wire to be twisted. As shown in FIG. 2, eachwire to be twisted leads from a reel 34 through a tensioning device andstop motion 36 to the pre-twister assembly 27. Between the creel 35 andpre-twister assembly 27 may be located a guide or lay plate 38, whichmay be supported on frame 12 by appropriate brackets, or may have itsown support.

The twisted strand from the pre-twister enters the front end of hollowshaft 55, passes therethrough and The pre-twister assembly 27 comprisesa hollow shaft 41 mounted for rotation in bearings 42 and 43. Shaft 41is provided with radially opposed and axially displaced ears 44 and 45to which are rotatably mounted sheaves 46 and 47 respectively. Sheaves46 and 47 through slots in shaft 41. to its hollow interior. The wire tobe twisted enters hollow shaft 41, preferably through a closing die 51,passes through the hollow interior to sheave 47, around sheave 47 andacross the hollow interior to sheave 46, around sheave 46 and back intothe hollow interior, and thence continues through the hollow interiorexiting at 52.

The main spindle and flyer assembly, or primary twister, 25 comprises ahollow shaft 55 rotatably mounted in a bearing 56 substantially coaxialwith hollow shaft 41 of the pre-twister assembly 27. A flyer 57 is 'afixed to and rotates with hollow shaft 55. Flyer 57 comprises a baseportion 58 and longitudinally extending arm 59 having a proximal end 60and a distal end 61. The proximal end 60 is afiixed to the base portion58. Coupled to the base portion 58 and positioned substantiallycoaxially with hollow shaft 55 is a grooved capstan or roller 61. Asecond capstan or roller 62 is rotatably mounted to the base portion 58at a location opposite the longitudinally extending arm 59. The flyer 57may also be provided with a counter-weight 63 located radially outwardfrom grooved roller 62 that cooperates with grooved roller 62 tocounterbalance longitudinally extending arm 59.

over a small pulley 66 to one groove of grooved roller ,62. The twistedstrand then makes a plurality of turns around grooved rollers 61 and 62,travels radially outward to a small pulley 67 mounted near. the proximalend of arm 59, travels along arm 59 to a small pulley 68 mounted at thedistal end of arm 59, and fromsmall pulley 68 is wound up on the take-upreel 23. As noted above, take-up reel 23 is mounted on take-up shaft 22which is rotated and reciprocated by a conventional drive means locatedin housing 21.

The driving torque for the primary twister is supplied by the twistedstrand being pulled around the grooved rollers 61 and 62 by the take-upreel 23. The number of revolutions of the primary twister per unitlength of twisted strand taken up on reel 23 is determined by aselectable gear ratio coupling (not shown) between grooved roller 61 andflyer 57. The pre-twister 27 is driven by a DC motor 71 having a gear 72mounted on its shaft 73. Gear 72 engages a gear 74 mounted on hollowshaft 41. By controlling DC motor 71 so that the pre-twister 27 isdriven at a rotationalvelocity greater than that of the primary twister25, the wires are first over twisted by the pre-twister 27 and thenpartially untwisted, or relaxed, between the pre-twister 27 and theprimary twister 25.

DC electrical signals proportional to or indicative of the velocities ofthe primary twister 25 and the pretwister 27 are provided by two DCtachometer generators 77 and 78. Tachometer generator 77 is coupled tohollow shaft 55 of the primary twister by gears 79 and 80. Tachometergenerator 78 is coupled to the pretwister 27 by a gear 81 that engagesgear 74. Thus, tachometer generator 77 provides a DC signal proportionalto the rotational velocity of the primary twister and tachometergenerator 78 provides a DC signal proportional to the rotationalvelocity of the pre-twister 27 Precise control of DC motor 71 tomaintain the velocity of the pre-twister in a desired preselected ratioto the velocity of the primary twister is accomplished by the servodriveillustrated diagrammatically in FIG.

3. Referring now to FIG. 3, there is shown diagrammatically a housing 21which encloses a main motor control 83, the main motor 84, and aclutch/brake 85. The main motor control 83 is actuated by a startpushbutton 86 and a stop push button 87. The torque transmitted by theclutch/brake 85 is controlled by a clutch control 88. Thus, as showndiagrammatically in FIG. 3, the main motor 84 drives the take-up reels23 through a clutch/brake 85 that is controlled by clutch control 88.Clutch control 88 regulates the torque transmitted by clutch/brake 85 toeffect a smooth controlled start-up of the twister to avoid breaking anyof the wires or strands. As each take-up reel 23 rotates andreciprocates it winds up twisted strand from its as sociated primarytwister 27. As explained above, the strand passing around groovedrollers 61 and 62 causes the primary twister to rotate. After startup,the take-up shafts 22 are driven at a constant rotational velocity.Thus, the more strand that is taken up on a reel 23, the faster itsprimary twister 25 will rotate.

Referring to FIG. 3, the rotational velocity of primary twister 25 isdenoted by 6 Tachometer generator 77, driven through gears 79 andgenerates a voltage E that is proportional to 0,. Thus, letting Krepresent the combined transfer function of tachometer generator 77 andthe gear ratio between gears 79 and 80, we may write E 1 K 6 volts perRPM. The voltage E may be selectively attenuated within a predeterminedrange by an infinitely variable ratio selector 90, which may comprise apotentiometer having a calibrated control knob to produce a voltage Eproportional. to E Letting K represent an infinitely variableattenuation factor or transfer function of ratio selector 90, we maywrite E K E Tachometer generator 78 produces a voltage E proportional tothe rotational velocity of pre-twister 27. Letting the combined transferfunction of tachometer generator 78 and the ratio of gears 74 and 81 berepresented by K and the rotational velocity of pre-twister 27 berepresented by 9 we can write E K 0 Voltages E and B are algebraicallycombined at a summing point 91 to create an error signal, 2 E E Thesumming point 91 compares signals (E and E indicative of the velocitiesof rotation of the primary and neutral strand, voltage E is equal tovoltage E and I there is no error signal e.

The error signal e provides the input to compensation network 92, theoutput of which is designated as E Letting l-l represent the transferfunction of compensation network 92, we can write E H e. E is used asthe input signal to a DC power amplifier 93, the output of which isdesignated as V,,,. Letting K 'represent the transfer function of poweramplifier 93, we can write, V K E Finally, V,,, is the armature drivingvoltage for DC motor 71, which taken with primary twister 25 has atransfer function H Thus, we can write 0 H V Combining the aboveequations, it will be seen that the closed loop transfer function may bewritten which will be recognized by those skilled in the art as being ofthe form where A is the open loop gain, B the feedback loop gain, and Kthe setpoint multiplier.

Because K is fixed by the physical properties of tachometer generator 77and the ratio of gears 79 and 80, the magnitude of H s), or in otherwords the steady state ratio of 49 /9 will be a function of theattenuation factor K of the infinitely variable ratio controller 90.Thus, by varying the setting of ratio controller 90, any desired ratioof 0 /0, may be selected.

One must be able to set controller 90 to the required ratio between thevelocity of the pre-twister 27 and the velocity of primary twister 25 inorder to provide a straight, neutral strand which is dead and which hasa desired lay. For given physical characteristics of the wire making upthe strand, the primary twister 25 must untwist or relax the strand by aspecific amount in order to have a truly dead or neutral strand whichwill remain straight without any inherent tendency to twist whenreleased. Or, what is the same thing, the pretwister 27- must overtwistthe strand by a specific amount in order to achieve a dead strand at thelay determined by the primary twister 25. If the pre-twister imparts toomuch overtwist to the strand, then the finished strand will tend totwist itself up into a tighter twist. If the pre-twister imparts toolittle overtwist to the strand, then the finished strand will tend tountwist itself to a looser twist.

The extent to which a particular strand must be overtwisted to provide astraight, neutral or dead" strand will vary depending upon the physicalcharacteristics of the wires making up the strand. Therefore, it

dary twister 27 and primary twister 25, i.e., ti /0 Thus.

if the ratio selector is not adjustable to provide the particularvelocity ratio necessary between the primary and secondary twisters 25and 27, the strand will not be truly straight and neutral but will havean inherent resilient tendency to twist in one of two directions. Toenable the twister assembly to form a strand which is truly dead orneutral when released so that the strand will lay straight and will nottend to twist in one 1 direction or the other, the ratio selector 90 isinfinitely variable throughout an operating range of approximately 0.8to 1.2 to select a desired ratio between the velocity of the pre-twister27 and primary twister 25. Since the ratio selector 90 is operable toselect on of an infinite number of velocity ratios within the operatingrange, the extent to which the pre-twister 27 overtwists the strand canbe carefully adjusted so that the finished strand will be truly dead orneutral.

In addition to providing an infinitely variable speed ratio between thepre-twister 27 and primary twister 25, it is important that the selectedspeed ratio be maintained with a high degree of accuracy. In onespecific embodiment of the invention the servodrive illustratedschematically in FIG. 3 maintained the velocity of the pre-twister 27 towithin one revolution per minute of the velocity corresponding to theselected velocity ratio through a range of primary twister speeds offrom 0 to 4,000 RPM. Thus, once the infinitely variable ratio selector90 has been adjusted to provide a desired velocity ratio between theprimary twister 25 and pretwister 27, this velocity ratio is accuratelymaintained during operation of the twister so that the resulting strandis truly dead or neutral. If the velocity ratio between the primary andsecondary twisters 25 and 27 varies by even a relatively small amountfrom the precise velocity ratio required to produce a neutral strand,the strand will no longer be neutral or dead. Therefore, the formationof a neutral or dead strand requires that the infinitely adjustableratio selector 90 be set to provide the exact velocity ratio between theprimary and secondary twisters 25 and 27 to overtwist the strand to theextent necessary for it to lay straight and dead when it is released andthat this ratio be maintained during operation of the twister assembly.

4 c l 2 A Bode plot of H; is shown in FIG. 4. It will be seen that nearzero frequency H has a valueK As will be understood by those skilled inthe art, around a frequency of l/T the magnitude of H begins decreasingat a rate of 3 db per octave. At a frequency of l/T the rate increasesanother 3 db per octave. In order to improve the accuracy of theservodrive of FIG. 3, it isdesirable to increase the loop gain aroundzero frequency as much as possible without reaching instability.Graphically, increasing forward gain without compensation will move theBode plot of H vertically upward from its position as shown in FIG. 4.This indicates that the gain in the frequency range of l/T to l/T andabove is increased in the same proportion as the gain around zerofrequency. Because a small increase in loop gain at the higherfrequencies will quickly lead to an oscillatory or unstable condition,the allowable increase in loop gain is severely limited.

In order to overcome the above described limitations, a compensationnetwork 92 was devised. This network is shown in detail in FIG. 5 andcomprises an input resistor R connected to the inverting input of anoperational amplifier 94 and a feedback impedance comprising theparallel combination of a capacitor C and a resistor R the combinationbeing connected in series with a resistor R between the output ofoperational amplifier 94 and its inverting input. The transfer functionof the compensation network of FIG. 5 may be written as He: 2+ a) Itwill be seen that this of the general fotm By properly selecting thealues of C, T is madeequal to T Thus, the above equation becomes It willbe seen that the effect of the compensation net:

work is to eliminate the break frequency 'at l/T and create a new breakfrequency at .1/T From zero 8 frequency up to approximately I lT H has amagnitude of (R R3)/R At frequency lfl the magnitude of H beginsdecreasing at a rate of 3 db per octave until it reaches frequency l/Twhere the magnitude is R /R,. Further increase in frequency does notchange the magnitude of H The inclusion of compensation network 92 inthe servo loop permits a very large increase in loop gain around zerofrequency without increasing. the gain above frequency 1 /T Thus, thegoal of greatly increasing the loop gain around zero frequency withoutcausing instability has been achieved.

In view of the foregoing description, it is apparent that the servodn'veillustrated schematically in FIG..3 is adjustableto provide for aninfinitely variable velocity ratio between the primary twister 25 andpre-twister 27. The selected velocity ratio between the pre-twister 27and primary twister 25 ismaintained with a very high degree of accuracyby providing the servodrive with a high loop gain around zero frequency.This high loop gain enables a relatively small error (e) to be correctedso that the velocity ratio is accurately maintained between the primarytwister 25 and pre-twister 27. This high degree of accuracy is necessaryin order to provide a truly straight and neutral strand.

If the velocity ratio varies from the selected velocity ratio, thestrand will have an inherent elastic tendency to twist and willtherefore be alive and will be relatively difficult to handle. Byproviding an infinitely variable velocity ratio selector in combinationwith the compensation network 92, the velocity ratio between the primarytwister 25 and secondary twister 27 can be accurately selected andmaintained in order to provide a truly dean or neutral strand. If theratio selector 90 was not infinitely variable, an operator would beunable to select the precise velocity ratio between the pretwister 27and primary twister 25 to provide a truly dead orneutral strand. Oncethe correct velocity ratio has'been selected with a high degree ofaccuracy, the velocity ratio is accurately maintained during operationof the primary and secondary twisters 25 and 27 by the compensatedservodrive in order to continuously produce a truly dead, or neutralstrand.

In the multispindle embodiment of the twister shown in FIG. 1, twocontrol panels 96 are provided, one on each side of frame 12. On eachcontrol panel is located a start pushbutton 86, stop pushbutton 87, anda clutch control 88 so that the main drive may be started and stoppedfrom either side of the twister. Also located on each control panel aretwo ratio selectors 90, one for each spindle on the same side of thetwister as the control panel.

It has been our intention to describe our invention in full, clear,concise, and'exact terms that would enable any person skilled in the artto practice it. We have set forth the best mode we have contemplated ofcarrying out our invention in the foregoing detailed description of thepresently preferred embodiment. Changes in the details of constructionmay be resorted to without departing from the'spirit of the invention.For example, the pre-twister could be driven by a constant speed driveand the primary twister be driven by a servodrive to accomplish the sameend.

Having describeda specific preferred embodiment of the invention, thefollowing is claimed:

1. Apparatus for twistirig wires to form a neutral strand, saidapparatus comprising means for supplying a plurality of wires, secondarytwister means for twisting the wires to form an overtwisted strand whichhas a resilient tendency to twist upon being released, said secondarytwister means including a rotatable'secondary twister assembly forengaging the plurality of wires and secondary drive means for rotatingsaid secondary twister assembly at a first velocity to overtwist thewires engaged by said secondary twister assembly to form the overtwistedstrand, primary twister means for partially untwisting the wires of theovertwisted strand to form a neutral strand, said primary twister meansincluding a rotatable primary twister assembly for engaging theovertwisted strand and primary drive means for rotat-v ing said primarytwister assembly at a second velocity which is less than said firstvelocity to untwist the overtwisted strand to an extent which is afunction of the difference between said first and second velocities, andcontrol means for maintaining a preselected ratio between said first andsecond velocities to control the extent to which said primary twistermeans untwists the overtwisted strand during operation of saidapparatus, said control means including means for providing a firstsignal indicative of said first velocity, means for providing a secondsignal indicative of said second velocity,

pensator means for improvin g the response of said control means andsaid one drive means to said error signal I to accurately control thevelocity of one of said twister assemblies to substantially maintainsaid preselected ratio.

2. Apparatus as set forth in claim 1 wherein said compensator meansincludes amplifier means for amplifying said error to a relatively largeextent when the difference between said first and second velocitiesvaries from said predetermined difference by a relatively small amount.

3. Apparatus as set forth in claim 1 wherein said means for providingsaid first signal includes first signal generator means driven by saidsecondary drive means at a rate which is proportional to said firstvelocity, and said means for providing said second signal includessecond signal generator means driven by said primary drive means at arate which is proportional to said second velocity.

4. Apparatus as set forth in claim 1 wherein said selector meansincludes infinitely variable ratio adjustselector means for preselectingthe ratio to be maintained between said first and second velocities todetermine the extent to which said primary twister means untwists theovertwisted strand to enable neutral strands to be formed by twistingwires having different characteristics, means for comparing said firstand second signals and for providing an error signal to one of saiddrive means if the ratio between said first and second velocities asindicated by said first and second signals is other than the preselectedratio, and compensator means imena ginr s ment means for infinitelyvarying the ratio between said first and second velocities tov enablesaid apparatus to be adjusted to form a neutral strand from wires ofdifferent physical characteristics.

5. Apparatus as set forth in claim 1 wherein said secondary drive meansincludes a motor for driving said secondary twister assembly and whereinsaid comsignal to vary the rate of operation of said motor to vary thefirst velocity relative to the second velocity.

6. Apparatus as set forth in claim 1 wherein said compensator meansincludes lead-lag compensation means that permits increased forward loopgain around zero frequency to increase system accuracy without thelnlnln In An

1. Apparatus for twisting wires to form a neutral strand, said apparatuscomprising means for supplying a plurality of wires, secondary twistermeans for twisting the wires to form an overtwisted strand which has aresilient tendency to twist upon being released, said secondary twistermeans including a rotatable secondary twister assembly for engaging theplurality of wires and secondary drive means for rotating said secondarytwister assembly at a first velocity to overtwist the wires engaged bysaid secondary twister assembly to form the overtwisted strand, primarytwister means for partially untwisting the wires of the overtwistedstrand to form a neutral strand, said primary twister means including arotatable primary twister assembly for engaging the overtwisted strandand primary drive means for rotating said primary twister assembly at asecond velocity which is less than said first velocity to untwist theovertwisted strand to an extent which is a function of the differencebetween said first and second velocities, and control means formaintaining a preselected ratio between said first and second velocitiesto control the extent to which said primary twister means untwists theovertwisted strand during operation of said apparatus, said controlmeans including means for providing a first signal indicative of saidfirst velocity, means for providing a second signal indicative of saidsecond velocity, selector means for preselecting the ratio to bemaintained between said first and second velocities to determine theextent to which said primary twister means untwists the overtwistedstrand to enable neutral strands to be formed by twisting wires havingdifferent characteristics, means for comparing said first and secondsignals and for providing an error signal to one of said drive means ifthe ratio between said first and second velocities as indicated by saidfirst and second signals is other than the preselected ratio, andcompensator means for improving the response of said control means andsaid one drive means to said error signal to accurately control thevelocity of one of said twister assemblies to substantially maintainsaid preselected ratio.
 2. Apparatus as set forth in claim 1 whereinsaid compensator means includes amplifier means for amplifying saiderror to a relatively large extent when the difference between saidfirst and second velocities varies from said predetermined difference bya relatively small amount.
 3. Apparatus as set forth in claim 1 whereinsaid means for providing said first signal includes first signalgenerator means driven by said secondary drive means at a rate which isproportional to said first velocity, and said means for providing saidsecond signal includes second signal generator means driven by saidprimary drive means at a rate which is proportional to said secondvelocity.
 4. Apparatus as set forth in claim 1 wherein said selectormeans includes infinitely variable ratio adjustment means for infinitelyvarying the ratio between said first and second velocities to enablesaid apparatus to be adjusted to form a neutral strand from wires ofdifferent physical characteristics.
 5. Apparatus as set forth in claim 1wherein said secondary drive means includes a motor for driving saidsecondary twister assembly and wherein said compensator means isoperable in response to said error signal to vary the rate of operationof said motor to vary the first velocity relative to the secondvelocity.
 6. Apparatus as set forth in claim 1 wherein said compensatormeans includes lead-lag compensation means that permits increasedforward loop gain around zero frequency to increase system accuracywithout the system becoming unstable.